The diagonal of the 18x24mm half frame is 30mm; the diagonal of the 24x36mm full frame is 43mm; so the “normal” focal length for a half frame would be about 30/43 x 50 or 35mm. The “35mm equivalent” of the 28mm on the Pen would be 43/30 x 28 or 40mm.
https://www.photo.net/discuss/threads/lenss-focal-length-for-half-frame-camera.72488/ #half_frame #equivalent
